The present invention provides a process for removing gas contaminants, such as hydrogen sulfide and carbon dioxide, from gas mixtures which include these contaminants. More particularly, the invention provides a process which utilizes a solvent comprising dialkyl ethers of polyethylene glycols and water to remove gas contaminants from product gas streams, such as natural, synthesis or other product gas streams.
It is known in the art to use solvents comprising mixtures of dialkyl ethers of polyalkylene glycols to remove gas contaminants from valuable product gases. These contaminants are removed by contacting the contaminated product gas with fresh solvent in an absorber or other specialized equipment operated under conditions of high pressure and/or low temperature which are favorable for absorption. Once the contaminants are removed, the decontaminated gas is ready for sale or for additional downstream conditioning, depending on the product stream specifications. The solvent is regenerated for reuse by driving off the absorbed contaminants under low pressure and/or high temperature conditions favorable for desorption. Flash tanks and/or stripper columns are typically used to effect this separation.
The use of dialkyl ethers of polyethylene glycols to remove gas contaminants commonly results in the co-absorption of valuable product gas, methane for example. The co-absorbed gases generally emerge from the solvent during regeneration and generally have little or no value because of the high concentration of undesirable contaminants. The loss of valuable co-absorbed gas is typically reduced by adding a high pressure recycle loop to the system design. The high pressure recycle loop allows partial regeneration of the rich solvent, and the gases liberated in the recycle loop, a mixture of valuable product gas and undesirable gas contaminants, are re-compressed, cooled and recycled back to the absorber where the liberated product gas is recovered. As is well understood in the art, the term "high pressure" is used to characterize the loop because the flash tank included within the loop operates at a pressure below the absorber pressure but above the pressure at which the solvent is finally regenerated.
While a high pressure recycle loop increases product gas revenue, capital and operating expenses for systems including a recycle loop are also increased. The high pressure flash tank, compression equipment and heat exchangers required for the recycle loop all have associated costs. In addition, increased gas flow through the absorber often increases the absorber circulation requirement, which necessitates larger high pressure pumps, heat exchangers, piping and other components throughout the entire system.
Economic optimization between increased product gas recovery and increased capital and operating costs is commonly attempted by adjusting the recycle flash tank pressure. High flash tank pressures result in low recycle gas volumes, low product gas recovery and low additional costs. Low flash tank pressures result in high recycle gas volumes, high product gas recovery and high additional costs. However, properly balancing these parameters has proven to be quite difficult, and adjusting the flash tank pressure has generally not provided an effective means of offsetting the increased capital and operating expenditures associated with systems which include a high pressure recycle loop and the need to maximize product gas recovery.